An inverter stops a PWM (Pulse Width Modulation) output within several ms when a power failure occurs in an inputted power source. At this time, it takes a long time to accelerate a load at the time of power restoration when the load has a great inertia, which may cause a great loss to industrial sites, such that a voltage dip or sag compensation technology for an inverter is applied to where a serious damage is expected by process failure when the inverter is stopped.
FIGS. 1a and 1b are schematic views illustrating a momentary voltage dip compensating operation in a conventional inverter, where FIG. 1a illustrates a case of normal state, while FIG. 1b illustrates a case where a power interruption is generated.
Referring to FIGS. 1a and 1b, an electrolytic condenser (210, illustrated outside of an inverter for convenience of explanation) embedded in an inverter (200) is normally charged with a power (100) from the inverter during a normal state (FIG. 1a), but drives a load (300) using the power charged in the electrolytic condenser (210) when the power is interrupted during power failure (FIG. 1b).
At this time, the conventional electrolytic condenser (210) is so designed as to secure 16 ms for a momentary voltage dip time, and when the momentary voltage dip time is within the 16 ms, the inverter (200) can drive the load (300) without stoppage. However, in view of the fact that the inverter (200) is designed to cope with the momentary voltage dip within the 16 ms, an irregular power supply area may be generated with a power failure for more than 16 ms, resulting in creation of problems in inverter stoppage and thereby leading to a great damage to industrial sites.
Meantime, current trends are that demands for energy saving and for medium voltage inverters as well are increasing, and Cascade H-Bridge (CHB) type inverters are largely used for the medium voltage inverters. Reliability is important for CHB type inverters because the CHB type inverters are mainly installed on important facilities in the industrial sites.
However, the voltage dip compensation method in the conventional inverter as illustrated in FIG. 1a has a problem that cannot overcome the momentary power failure if applied to the CHB type medium voltage inverters, the reasons of which may be as follows:
First, the conventional momentary voltage dip compensation method cannot control a DC link of a plurality of unit power cells of a medium voltage inverter; Second, in the conventional momentary voltage dip compensation method using a feed-backed reference voltage as a voltage command of DC link, when the method is actually applied to a medium voltage inverter, the DC link voltage of each power cell cannot be driven as one voltage command due to parasitic components possessed by a capacitor; and
Lastly, the conventional momentary voltage dip compensation method has failed to provide a solution in consideration of external environment of a large load mounted on CHB type medium voltage inverters. The conventional momentary voltage dip compensation method is fraught with a disadvantage of finding a deceleration gradient that generates power source regeneration. The conventional momentary voltage dip compensation method is such that a deceleration time that generates regeneration must be found in advance by changing the deceleration time during a normal operation, and there is a possibility of generating a trip instead of normal operation due to lack of regeneration amount, when the deceleration time that generates the regeneration is within 10 sec.